Arrange the elements according to the increasing order of first ionization energies.

Na, Mg, Ca, Ba

The ionization energy of elements decreases down the group and increases across the period. Due to this Mg has higher ionization energy than Na as Mg belongs to period 3 and group 2. As Mg, Ca and Ba belong to the same group, then ionization energy will decrease from Ca to Ba.

So, the order of ionization energy is Ba<Ca<Na<Mg.